FIG. 1 illustrates a nacelle 3 which houses a gas turbine engine (not shown) and which is suspended from a wing 6 of an aircraft. It is important to measure characteristics of the incoming airstream 9 such as the airstream's temperature, pressure, and velocity. For this measurement, various types of probes have been devised and they typically extend from the inner barrel 12 of the nacelle 3 as illustrated by probe 15.
FIG. 2 is an enlarged view of the probe 15. Typically, the sensing element 18 of the probe 15 is supported by a mast 21 in order to position the sensing element 18 outside the boundary layer 24 in order to reduce the effects of the boundary layer 24 upon the measurement taken. In general, the boundary layer 24 has vastly different characteristics than the incoming airstream 9 (sometimes called freestream air) in FIG. 1 and so measurement of the boundary layer 24 is to be avoided. For example, one different characteristic results from the fact that some engine nacelles contain apparatus which heat the inner barrel 12 to a few hundred degrees F. in order to inhibit ice formation. This heating artificially alters the temperature of the boundary layer 24, thus causing its temperature to significantly differ from that of the incoming airstream 9 generally.
Irrespective of boundary layer considerations, other problems are faced by the probe 15. One, the probe 15 is apt to be struck by incoming objects such as birds, insects, ice particles, and miscellaneous types of debris. Two, rainwater will certainly impinge upon the sensing element 18. Rainwater tends to alter measurements taken by temperature probes.
Even apart from the above considerations, the incoming airstream 9 is not always parallel to the centerline 27 of the nacelle 3 as shown in FIG. 1. For example, during takeoff, the incoming airstream more closely resembles that shown by arrow 30. A pressure sensor contained in probe 15 in FIG. 2 will give different readings depending upon the direction at which it is impinged by the incoming air. That is, airstreams shown by arrows 33A-B in FIG. 2, which are otherwise identical, will give different pressure readings simply because of the different directions from which they impinge upon the probe 15. It is generally desirable that the angle of attack which an airstream makes upon a pressure sensor be held constant.